The electromagnetically stirring of the molten steel in the mold for the continuous casting (hereinafter referred to briefly as "mold") is generally aimed at the improvement on the surface defects of the surface portion of the continuously cast slab, particularly improvement in the quality of the solidification structure, and occasionally is aimed at enlargement of the kinds of steels to be cast (such as slightly deoxidized steel), and its application has recently been widely tried.
With respect to the direction in which the molten steel in the mold is electromagnetically stirred, the stirring is broadly classified into horizontal stirring and vertical stirring.
The horizontal stirring forcedly causes the molten steel in the mold to flow in a horizontal plane, and the horizontal circulation flow as shown in solid lines in FIGS. 1a and 1b or one way parallel flow as also shown by solid lines in FIG. 1c is produced in the bath surface of the molten steel in the mold.
On the other hand, the vertical stirring forcedly causes the flow of the molten steel in the mold along the height direction of the mold (that is, as shown by arrows of FIGS. 2a and 2b) as if the flow would be to a rimming action in the ingot making process of a rimmed steel (hereinafter referred to as rimming flow) or occasionally in a direction opposite thereto.
The direction in which the molten steel flows is generally shown by the arrow 1 in each of the figures, in which a reference numeral 2 is a mold, a reference numeral 3 a molten steel in the mold, and a reference numeral 4 a linear motor type stirrer. The direction of the electromagnetically stirring thrust is shown by white arrows 5, and a reference numeral 6 is an immersion nozzle.
The directional selection of either one of the horizontal stirring and the vertical stirring is made depending upon the stirring purpose and power to be applied.
The ordinary profile of the continuously cast piece such as the slabs is 200-300 mm in thickness and 1,000-2,500 mm in width. The height of the mold 2 serving for the continuous casting is ordinarily 700-900 mm in height, and therefore the width of the mold 2 reaches approximately 2-3 times as large as the height thereof.
Therefore, from the standpoint of the stirring efficiency, the horizontal stirring of FIG. 1 in which the distance for acceleration of the stirrer 4 can be made larger is more advantageous as compared with the vertical stirring of FIG. 2, and the stirring speed can be made larger in the former case. However, since stagnation is likely to be produced at both the ends in the width direction of the mold 2, that is, in the vicinity of the corners of the short sides, there is the liklihood that bubbles and non-metallic inclusions are accumulated near the corners.
To the contrary, the vertical stirring is inferior in terms of the stirring efficiency as mentioned above, but is more advantageous in that the rimming flow directly serves to float the bubbles and the inclusions. The concentrated stagnation is relatively hard to be formed, and it can be said that since the vicinity of the meniscus becomes the termination point of the acceleration, the vertical stirring is suitable for mainly stirring the vicinity of the meniscus.
From the foregoing, the general adoption standard for the stirring of the molten steel in the mold can be summarized as follows:
When the stirring flow speed is intended to be large and particularly the effect of cleaning off the bubbles and the inclusions at the solidification interface is intended, the horizontal stirring, particularly in a swirling fashion, is well suited, while the vertical stirring is suitable for the purpose of improving the quality of the surface portion through stirring the vicinity of the meniscus and controlling the flowing in the mold.
As mentioned above, the stirring directions both have their own merits and demerits, and when in application, it is necessary to select such a stirring system as is suited for the kind and the composition of a steel and the casting conditions in each operation.
Therefore, since the continuously casting apparatus to which is applied the conventional electromagnetic stirring of the molten steel in the mold is restricted to a single stirring system despite it treats various kinds of steels, the advantage of improving the quality can not be fully exhibited thereby.
Further, even through it is preferably that both horizontal and vertical stirring systems are selectively used depending upon the kinds of the steels and the casting conditions, as shown in FIGS. 1 and 2, the conventional system stirrers have the width extended over substantially the whole width of the mold 2, so that the dimensions of the stirrer in the width direction and the height direction largely differ from each other and the installation direction of the stirrer can not be changed. Further, even if the flow can be oppositely switched in the horizontal direction or in the vertical direction by electrical operation, the directional change by 90.degree. from the horizontal direction to the vertical direction or from the vertical direction to the horizontal direction can not be made. Thus, the conventional stirring is fixed to either the horizontal direction or vertical direction.